It is known that correction of certain optical defects can be accomplished by imparting non-spherical corrective characteristics to one or more surfaces of a contact lens such as one or more of cylindrical, bifocal, or multifocal characteristics. The use of these lenses is problematic in that the lens must be maintained at a specific orientation while on eye to be effective yet, once the lens is on-eye, it will rotate on the eye due to blinking as well as eyelid and tear fluid movement.
Maintenance of the on-eye orientation of a lens typically is accomplished by altering the mechanical characteristics of the lens. For example, prism stabilization, including without limitation decentering of the lens' front surface relative to the back surface, prismatic balancing, thickening of the lower lens edge, supporting the lens on the lower eyelid, forming depressions or elevations on the lens' surface, and truncating the lens edge, has been used.
Additionally, dynamic stabilization methods are known including dual thin zone designs. In dual thin zone designs, the lens is stabilized by reducing the thickness in certain areas of the lens' front, or object side, surface. More specifically, thin zones are provided at each of two symmetrically lying regions along the 90-270 degree axis on the lens' front surface periphery. In addition to the thin zones, two thickened regions form the remainder of the lens' periphery and are present along the horizontal, or 0-180 degree axis.
The dual thin zone design is disadvantageous because it is prone to excessive rotation when placed on the eye, which complicates fitting of the lens. Additionally, the design is unpredictable as to on eye rotation because the rotation varies with variations in eye shape, lid geometries, and the lens wearer's blinking. This also complicates, or makes impossible, fitting of the lens. Thus, there is a need for dual thin zone designs that overcome some of these disadvantages.